The present invention broadly concerns devices for use in spinal implant systems, particularly those using spinal rods contoured for connection at various locations along the length of the spinal column. More specifically, the invention concerns a device for fixing the spinal rod to a fixation element of the implant system which provides direct engagement to a vertebra of the spinal column.
Several techniques and systems have been developed for use in correcting and stabilizing spinal curves and facilitating spinal fusion. In one system, a bendable rod is longitudinally disposed adjacent the vertebral column and is fixed to various vertebrae along the length of the column by way of a number of fixation elements. A variety of fixation elements can be provided, such as hooks or bone screws, which are configured to engage specific portions of the vertebra.
An example of one such system is the TSRH.RTM. spinal system of Danek Medical, Inc. In this system, the hooks or screws are engaged to the spinal rod by way of eyebolts. As is well known in the art, the eyebolts are threaded onto the spinal rod and captured within yokes on the fixation hook or screw. A nut is threaded onto a threaded post of the eyebolt to clamp the yoke and rigidly fix the hook or screw element to the spinal fixation rod. Details of the TSRH spinal implant system are disclosed in the "Surgical Technique Manual" provided by Danek Medical, Inc., published in 1990, which disclosure is incorporated herein by reference.
It is the goal of the surgeon using such spinal implant systems to apply the vertebral fixation elements (hooks and/or screws) to the spine in the appropriate anatomic position, and then to engage each fixation element to the spinal rod. Once the spinal implant system is assembled it is then possible to correct anatomical deformities and stabilize the spine. In the ideal circumstance, the fixation elements are located in a colinear position substantially parallel to the vertebral column. However, in many circumstances particular vertebrae may deviate from this colinear position. Under these circumstances, it is often necessary to contour the rod in the saggital plane to account for abnormal lateral curvatures of the spine, such as scoloitic curvatures. After the rod has been contoured as dictated by the anatomy, the fixation hooks or bone screws can be engaged directly to the laterally offset vertebrae. This lateral offset of the vertebral column in the saggital plane can also be accommodated by a spinal fixation system of the type shown in the patent to Steffee U.S. Pat. No. 4,771,767, in which a number of smaller rods are engaged between lateral connectors. Other related systems provide means for laterally offsetting the screw from the spinal rod, such as systems shown in the patents to Steffee, U.S. Pat. No. 4,719,905; Howland, U.S. Pat. No. 4,653,481; and Frigg, U.S. Pat. No. 5,002,542.
One difficulty with spinal fixation systems of the prior art, and particularly those utilizing spinal rods, is that contouring the rod in the saggital plane can often be difficult, particularly since the rod must also be contoured to the normal cervical and lumbar curvatures of the spine. Three dimensional rod contouring is often physically difficult and is sometimes not possible in the space available for a particular patient.
To date, no spinal fixation system has been developed which adequately and simply addresses this problem by eliminating the need to contour the fixation rod in the saggital plane. The patent to Frigg '542 shows a pedicle screw clamp which is offset from the fixation rod and which includes means for varying the distance between the center of the rod and pedicle screw. In Frigg, a pair of splined surfaces is provided which are engaged by way of a special offset hook configuration. The Frigg device is not readily adapted to mount a variety of spinal fixation elements, such as bone screws or fixation hooks. Moreover, the Frigg pedicle screw clamp requires a specially configured hook to engage the spinal rod.
Consequently, there remains a need in the field of spinal fixation for a connector which acts as an intermediary component between the spinal rod and a vertebral fixation element, such as a bone screw or hook, to laterally offset the element from the spinal rod.